Currently the liquid crystal compounds are widely applied in various types of displays, electro-optical devices, sensors and the like. A large number of liquid crystal compounds used in display area, wherein the nematic liquid crystal is the most widely used. Nematic liquid crystals have been used in passive TN, STN matrix display and TFT active matrix system.
Although the technology of thin-film-transistor liquid-crystal display (TFT-LCD) has been mature, it should be continuously improved to decrease the response time, to widen the phase transition temperature, to decrease the driving voltage, and so on. It has been well-known that liquid crystals play an important role in improving the characteristics of LCDs.
The liquid crystal compounds are required to have high chemical and thermal stability, as well as high stability in electric field and electromagnetic radiation. The liquid crystal compounds used in TFT-LCD not only should be stable to heat, UV light, electric field, and electromagnetic radiation, but also have the wide temperature range of a nematic phase, appropriate optical anisotropy, very high resistance, high voltage holding ratio.
For dynamic picture display such as LCD-TV, in order to improve the characteristics of the device to reduce the picture motion blur and trailing, it is desired that the liquid crystal mixtures have appropriate physical properties with a short response time, a low rotational viscosity (γ1), and a low driving voltage. Also it is great significant to elevate the dielectric anisotropy (Δ∈) of the liquid crystal compound for the modulation of liquid crystal mixture.
It has been disclosed that the liquid crystal compounds containing difluoromethyleneoxy group (—CF2O—) have low rotational viscosity. In addition, the presence of —CF2O— group and fluorine atoms in the molecules results in the increase of the dielectric anisotropy (Δ∈). Merck of Germany and Chisso of Japan have reported several liquid crystal compounds containing various substituents and difluoromethyleneoxy group (—CF2O—) (CN1717468A, CN101143808A, CN101157862A). These compounds have good performance, but also have the disadvantage of low clearing point.